The best time to go for 4D ultrasound?

Rose • Fiancée & Mommy

I had a 4D ultrasound at 16 weeks. The baby looked like a skinny alien lol.

I wanna book another but it costs £80.00 so I'd like to go when the baby looks like a healthy baby so I can get some really good pics!

When's the best time in weeks to go? I'm 24 weeks now.